C++ daemon forking causes mysql errors

Question

I have a daemon that forks the process.

This daemon access a database using mysql connector library.

When I do not fork, I am able to open and read a database fine, however, when I fork, I get

MySQL server has gone away 

errors consistently on the first query...

Anyone know what could be causing this?

Answer 1

Edit Oh, my apologies for misinterpreting

Still the problems with differences between daemonized/non-daemonized are roughly with the following class of options:

• environment variables
  • LIBPATH
  • PATH
  • HOME, UID, EUID (HOME surprisingly enough gets (ab)used way too often)
  • mysql specific variables
• permissions
  • what user is the daemon running as? elevated or privilege separation?
• current working directory (traditionally / for daemons, where / might be a chroot jail instead of 'real' /)

• Starting with kernel 2.4.19, Linux provides per-process mount namespaces. A mount namespace is the set of file system mounts that are visible to a process. Mount-point namespaces can be (and usually are) shared between multiple processes, and changes to the namespace (i.e., mounts and unmounts) by one process are visible to all other processes sharing the same namespace. (The pre-2.4.19 Linux situation can be considered as one in which a single namespace was shared by every process on the system.)

• detached stdin/stdout causing trouble (IMO that would mean badly designed library, but who am I)

• watch it that specific resources (file locks, socket connections, threads (!)) are NOT inherited across fork/execve. I recommend reading the linked on daemonization (below), especially for example the section on 'Mutual Exclusion and Running a Single Copy [open,lockf,getpid]'

I'm sure I'm forgetting stuff

Ermm... what are you starting a mysql server process for? Mysql has plenty of sound init scripts that do work.

On the subject of proper daemonization: http://www.enderunix.org/docs/eng/daemon.php

Pay attention to the effects of sharing resources with fork children (e.g. file descriptors).

Besides that, you could just be missing basic environment settings. Peruse the official init scripts for mysql to find out which you need.